Process for producing silver salt of benzotriazole

ABSTRACT

THE SILVER SALT OF BENZOTRIAZOLE HAVING A PARTICLE SIZE OF LESS THAN 5 MICRONS IN LENGTH IS PREPARED BY A SPECIFIC COMBINATION OF SOLVENTS USED FOR THE REACTION OF SILVER NITRATE AND BENZOTRIAZOLE IN SOLUTION. SILVER NITRATE IS DISSOLVED IN SOLVENT A WHICH IS MISCIBLE WITH NITRIC ACID WHICH MIGHT BE FORMED BY THE ABOVE REACTION. SOLVENT A MAY, FOR EXAMPLE, BE WATER, DIMETHYLFORMAMIDE OR DIMETHYLSULFOXIDE. BENZOTRIAZOLE IS DISSOLVED IN SOLVENT B WHICH IS A SOLVENT FOR BENZOTRIAZOLE, BUT WHICH DOES NOT SUBSTANTIALLY OR COMPLETELY DISSOLVE THE SILVER SALT OF BENZOTRIAZOLE. WHEN SOLVENT A IS WATER, SOLVENT B MAY, FOR EXAMPLE, BE TRICRESYL PHOSPHATE, DIMETHOXY ETHYL PHTHALATE, DI-N-BUTYL PHTHALATE, DIETHYL SEBACRATE, MONOOCTYL-DIBUTYL PHOSPHATE, TRIBUTYL PHOSPHATE OR CASTOR OIL. WHERE SOLVENT A IS DIMETHYL FORMAMIDE OR DIMETHYL SUFLOXIDE, SOLVENT B MAY, FOR EXAMPLE, BE COTTONSEED OIL, LINSEED OIL OR TSUBAKI OIL. SOLUTIONS OF SILVER NITRATE IN SOLVENT A AND BENZOTRIAZOLE IN SOLVENT B ARE MIXTED TO FORM THE SILVER SALT OF BENZOTRIAZOLE. THE PRODUCT IS WASHED WITH WASHING LIQUID C WHICH MAY, FOR EXAMPLE, BE METHANOL, ETHANOL OR ACETONE. TO THE REACTION SYSTEM THERE CAN BE ADDED A SURFACTANT TO MAKE THE SIZE OF THE OBTAINED PARTICLES UNIFORM.

0a. 11, 1 972 OHK B HAL 3,699,114

PROCESS FOR PRODUCING SILVER SALT OF BENZOTRIAZOLE Filed July 20, 1970 INVENTORS KlNJI OHKUBO JUNPEI NOGUCHI TAKAO MASUDA in A RM M BY Z; f

ATTORNEYS United States Patent s. or. 260-299 16 Claims ABSTRACT OF THE DISCLOSURE V The silver salt of benzotriazole having a particle size of less than 5 microns in length is prepared by a specific combination of solvents used for the reaction of silver nitrate and benzotriazole in solution. Silver nitrate is dissolved in solvent A which is miscible with nitric acid which might be formed by the above reaction. Solvent ;A may, for example, be water, dimethylformamide or dimethylsulfoxide. Benzotriazole is dissolved in solvent B which is a solvent for benzotriazole, but which does not substantially or completely dissolve the silver salt of benzotriazole. When solvent A is water, solvent B may, .for example, be tricresyl phosphate, dimethoxy ethyl phthalate, di-n-butyl phthalate, diethyl sebacate, monooctyl-dibutyl phosphate, tributyl phosphate or castor oil. Where solvent A is dimethyl formarnide or dimethyl sulfaox'ide, solvent B may, for example, be cottonseed oil,

,linseed oil or tsubaki oil. Solutions of silver nitrate in solvent A and benzotriazole in solvent B are mixed to form the silver salt of benzotriazole. The productis washed with washing liquid C which may, for example, be methanol, ethanol or acetone. To the reaction system there can be added a surfactant to make the size of the obtained particles uniform.

' CROSS-REFERENCES TO RELATED J APPLICATIONS This application is a continuation-in-part application of our copending application U.S. Ser. No. 629,963, filed Apr. 11, 1967, now abandoned. I

BACKGRQUND OF THE INVENTION l (1) Field of the invention I The present invention relates to a process for producing the silver salt of benzotriazole and more particularly, is concerned with a process for producing the silver salts of benzotriazole having a small particle size which are suitable for use in heat developable light sensitive materials.

" .(2) Description of the prior art In photographic light-sensitive materials Iformed with silver salts, it is desirable for economy that the quantity of a light-sensitive silver salt added to the light-sensitive layer be as small as possible to provide a photographic image having the desired density. V

"As disclosed in a well-known literature in the photographic chemistry, that is, The Theory of the Photographic Process by 'Mees, page 814, published in 1954 by MacMillan, New York, it is generally believed that as'the size of developed (reduced) silver particles 'is inversely proportional to their covering power, the size of silver particles formed by the reduction of crystal particles of silver salts during development, that is, silver 3,699,114 Patented Oct. 17, 1972 portional to their photometric constanttin a relation inversely proportional to the covering power). In ,other words, developed silver particles havinga small particle size give a larger covering power and higher density than large particles do at a constant weight. r v

On the other hand, it is generally believed thatdeveloped silver particles of a. coarse size tend to result from silver salt crystal particles of a coarse. size, while developed silver particles of a small size tend to result from silver salt crystal particles of a small'size'Therefore, it is desirable to obtain a desired image density, .to use a light-sensitive silver salt in an amount as small as possible to reduce the size of the silver salt particles. Making the particle size of a silver salt small is very important because various problems encountered with light- .sensitive materials using a silver salt crystal of. a coarse particle size can be solved, e.g., a large amount of silver salt must be used in order to obtain a desired image density and the resolving power and sharpnessof the resulting image are interior. I We, the inventors, have developed a heat-developable light-sensitive material in which a silver salt of benzotriazole, an inorganic halogen compound capable of forming a silver halide by reaction with the silver salt of benzotriazole, a material capable of forming a basic substance by. heating and reducing agent are incorported in a layer provided on a support. In this case it is also important, for the reasons above, to minimize the size of the silver salt of benzotriazole corresponding to the foregoing silver salt crystal. The silver salt of benzotriazole is prepared, in general, by mixing a solution of silver nitrate in water and a solution of benzotriazole in methanol and reacting silver nitrate and benzotriazole. However, when a combination of solvents well miscible with each other, such as water and methanol, is used, only coarse spindle-shaped or needle-shaped crystal particles of the silver salt of benzotriazole having a particle size of 10-100 microns in length are obtained.

In the prior art, in order to obtain silver salts of benzotriazole having a small particle size coarse crystal particles are pulverized for a long time in a ball mixer or homomixer, but the particle size of the crystal given thereby is at best about 10 microns in length, and a lightsensitive material using crystal particles of such a size cannot givedesired image density and resolving power. Furthermore, such processes are timeand labor-consuming.

SUMMARY OF THE INVENTION The inventors have made efforts to overcome the foregoing disadvantages of the prior art and found a new process for producing a silver salt of benzotriazole with 'a small particle size.

According to the present invention, there is provided aprocess for producing a silver salt of benzotriazole having a small particle size, which comprises mixing a solution of silver nitrate dissolved in a first solvent A, in which silver nitrate is soluble and nitric acid is miscible but in which the silver salt of benzotriazole is insoluble, with .a solution of benzotriazole dissolved in a second solvent 'B, in which benzotriazole is soluble but in which the silver salt of benzotriazole and silver nitrate are insoluble, said solvent A being miscible with solvent B in an amount of from 1 to 30% by weight of the total weight of solutions A and B, and reacting said silver nitrate and said benzotriazole to form the silver salt of benzotriazole.

By soluble and related language is meant that more than 0.5 g.," preferably more than 5 g. of the solute is dissolved in g. of a solvent, and by insoluble and solvent. By the term a small particle-size is meant a.

particle size of less than 5 1. in length, more preferably less than 1, in length.

As is mentioned above, the main feature of the present invention lies in the selection of a suitable combina- BRIEF DESCRIPTION OF THE DRAWING The single figure of the drawing represents the mutual solubility curves of solvents A and B and washing liquid C on trigonometric coordinates.

DETAILED DESCRIPTION OF THE INVENTION Solvent A of the invention dissolves silver nitrate, and is miscible with nitric acid formed by the reaction of silver nitrate ancl benzotriazole, but little or no silver salt of benzotriazole is dissolved therein. Illustrative of such solvents are water, dialkylformamides such as dimethylformamide and diethylformamide, and dialkylsulfoxide such as dimethylsulfoxide, diethylsulfoxide anddi-nor -iso-propylsulfoxide. Although it is most preferred that solvent A does not dissolve benzotriazole, some dissolving does not render the process inoperable,

but only lowers theyield of silver salt of benzotriazole.v

Such a solvent may be used as occasion demands.

Solvent B is capable of dissolving benzotriazole, but little or none of the silver salt of benzotriazole and silver nitrate.

Further, solvent A and solvent B must mutually have the relationship that the solubility of solvent A in solvent B is 1-30% by weight based upon the total weight of mixed solvents A and B. Examining practical solvents satisfying this relation, it is found that, in many cases, solvent A hardly dissolves solvent B. When the solubility of solvent A in solvent B is large, as in the foregoing combination of methanol and water, coarse crystals of silver salt of benzotriazole are formed, while when it is too small, for example, as in the combination of oil and water, and toluene and water, the silver salt of benzotriazole is not formed in a satisfactory yield. Therefore, suitable materials used as solvent B are phosphates, phthalates and. esters of dibasic acids as well as glycerine esters such as castor oil, cotton oil, linseed oil and tsubaki oil.

For practical combinations of both solvents, there are adopted as solvent B trialkyl phosphates such as triethyl phosphate, tributyl phosphate and octyl-di-butyl phosphate; triaryloxy phosphates such as tricresyl phosphate; and diesters of phthalic acid such as dimethyl phthalate, diethyl phthalate, dipropyl' phthalate, dibutyl phthalate, di-iso-butyl phthalate, dioctyl phthalate, di-Z-ethylhexyl phthalate, and dimethoxyethyl phthalate; diesters of sebacic acid such as dimethyl sebacate, dibutyl sebacate, and dioctyl sebacate; and castor oil when water, dimethyl formamide, and dimethyl sulfoxide are used as solvent A. There can also be used as solvent B cottonseed oil, linseed oil and tsubaki oil when dimethyl formamide and dimethyl sulfoxide are used as solvent A.

More desirable results can be obtained by adding a surfactant to the reaction system of the invention such that the reaction. between benzotriazole and silver nitrate can be homogeneously carried out, thereby causing an average particle size of the reaction product within the limited range.

As a surfactant 1or surfactant mixture, any surfactant,

preferably anionic or nonionic surfactants can be used. The purpose of sucha surfactant is to enable a homogeneous reaction system to be formed. Some specific surfactants that can be protonated'and used in the process of the invention include compounds of the following formula: AS O B, Where A can be an alkyl substituted aryl group preferably with l-4- carbon atoms in the-alkyl group, an alkoxy group, an alkyl amide-substituted alkylene group preferably with .1-4 carbon atoms, and an alkyl substituted carboxyalkyl group. B is a cation, preferably K or Na.

Such a surfactant may be added to the solution of silver nitrate or may be added to the solution of benzotriazole as an aqueous or alcoholic solution, onma'y be added upon mixing with the silver nitrate. solution simultaneously. The amount thereof to be added is 0.01 g. to 20 g. preferably 0.1 g. to 10 g., per 100 g. of the silver nitrate used.

In the selection of solvents, considering the solubility thereof as described above, it is further necessary to select a washing liquid C used for taking and washing the silver salt of benzotriazole formed after the reaction of silver nitrate and benzotriazole. This washing liquid .C is a solvent, preferably a volatile liquid miscible with solvent A or solvent B in any arbitrary ratio which is capable of forming a uniform ternary phase system containing more than 10% of solvents A and B respectively, and which is capable of dissolving little or none of the silver salt of benzotriazole (as, heretofore defined). Illustrative of such washing liquids are methanol, ethanol and acetone.

In selecting the solvents, it is quite helpful not only to know the solubilities of silver nitrate, benzotriazole, nitric acid and the silver salt of benzotriazole in each solvent, but also to determine the mutual solubility curves of solvent A, solvent B and washing liquid C on trigonometric coordinates. One of the examples is shown in the accompanying drawing, in which the upper region of the curve stands for a composition wherein the three materials form a homogeneous phase, while the lower region of the curve stands for a composition wherein there is phase separation. The range shown by the mark on the base line is the limit of mutual solubility of solvent A and solvent B, that is, the curves must cut the base line in this range, in order to obtain the desired fine crystals of the silver salt of benzotriazole.

Mark (9, shown near apex C, is the composition of the three component homogeneous 'phase described above. With reference to the properties to be endowed to washing liquid C, the curves must be lower than-this point.

Further, since washing liquid C can be mixed with solvent A in any desired ratio, the curve must not cross side AC and must form an asymptote with this side. In the drawing, curves I and II stand for the following cases, at 25 C.:

Solvent Washing A B liquid 0 I Water... T C P Methanol. II -do P Acetone.

so long as the reactants can be contacted in the liquid state.

The nitric acid formed by .the reaction of silver nitrate and benzotriazole is mainly present in the phase enriched with solvent A. In order to recover the fine crystals of the silver salt of benzotriazole by purifying the system, the phase enriched-with solvent A is removed by .de-

-cantation or any other method. The remaining phase,

enriched with solvent B containing the silver salt of benzotriazole, is washed repeatedly to a sufficient degree with solvent A orother solution. This phase is then sufliciently washed with washing solution C having the above-mentioned mutual solubility with solvents A and B. When the thus remaining silver salt of benzotriazole is dried to evaporate 01f washing solution C, a highly pure silver salt of benzotriazole is obtained. 1

When benzotriazole and silver nitrate are reacted using these combinations of solvents and adjusting the operating conditions such as concentration and temperature to same as those of the foregoing methanol-water system, spindle-shaped crystals of a silver salt of benzotriazole having a length of-0.13 microns are produced. These crystal particles are generally present as a solid phase dispersed in a solvent B-rich phase which has been separated from a solvent A-rich phase, while nitric acid formed by the reaction of silver nitrate and benzotriazole is mainly present in the solvent A-rich phase.

Fine crystals of the silver salt of benzotriazole are recovered from this system by removing the solvent A- rich phase by decantation or the like, washing repeatedly the remaining solvent B-rich phase containing a silver salt of benzotriazole with the solvent A or other liquid and then with a washing liquid C having a mutual solubility with, the solvents A and B as mentioned hereinbefore. The further remaining silver salt of benzotriazole is dried to vaporize the washing liquid C to obtain a silver salt-of benzotriazole of high purity.

When the fine crystals of a silver salt of benzotriazole thus obtained are applied to a heat-developable lightsensitive material as mentioned before, the disadvantages appearing in the case of coarse particles are overcome and a dispersion of crystal particles in any state of the art macromolecular colloid binder can be prepared by a simple physical method. The heat-developablelightsensitive material gives high density and high resolving power image. p

In accordance with the method of the invention, any

fine crystal particles can be obtained by suitably selecting solvents for the silver nitrate and benzotriazole having a suitable mutual solubility. with each other.

The mixing conditions of a solution of silver nitrate and a solution of benzotriazole are not limited to the foregoing embodiment, but the solution concentrations, solution temperatures, mixing speed and stirring condition may be optionally varied according to any desired particle diameter. Set forth below are several specific examples of this invention, illustrating the best methods of preparing the fine crystals of the silver salt of benzotriazole. I

' EXAMPLE 1 methanol therein, the dispersion 'was washed while filtering. After drying between two sheets of dried filter papers, 16 g. of the thus obtained silver salt was obtained, and the average grain size of the particles was 0.2. micron.

peated except that di-n-butyl phthalate was used as the 6 EXAMPLE 2 The procedure described in Example 1 was repeated except that tributyl phosphate (TBP') was used in place of TCP and methanol :Was used as a washing liquid. 17 g. of silver salt of benzotriazole having an average particle size of 0.25 micron was obtained. i I

Light-sensitive compositions having the following com ponents were applied to photographic papers "to make a heat-developable light-sensitive material using particles of a silver salt of benzotriazole prepared by the known meth- 0d using a solvent system of water and methanol and as obtained in Examples 1 and 2. Ethanol was used as the solvent."

Polyvinyl butyral 6.0 Silver salt 'of benzotriazole 1J2 Strontium iodide (hexahydrate) 0.055 Sensitizing dye* 0.0001 Hydroquinone 0.3 Sebacic acid 4.0

*3-ethyl-5rl (3 methyl 2 thlazoliuylidene) ethylidenplrhodanlne. The thus resulting light-sensitive material was exposed by the use of a tungsten lamp and heated at C. for 10 seconds to give an image. The image densities and resolving powers are tabulated below for comparison, from which it is evident that the maximum density ratio and resolving power are remarkably improved in the case of using the silver salt of benzotriazole prepared by the process of the invention.

I The maximum density ratio described hereinbefore means the ratio of the maximum density of the images obtained by the methods according to the present invention to that in the case of the conventional method.

Silver salt source- Known method Ex. 1 Ex. 3

Particle size. 30-70;; 0. 2 0. 25a Solvent of benzotriazole-methanol TCP TBP Maximum density ratio 1.0 l. 3 1. 6 2. 5 3. 2 4. 0

Resolving power, lines/mm...

Silver salt source- Known method Ex. 1 Ex. 3 Solvent fol benzotriazole TOP TBP Maximumdensity ratio 1. 0 2. 1 1. 5 Resolving power (lines/mm) 3.2 5. 0 4. 0

"1 Methanol.

" EXAMPLE 3 The procedure described in Example 1 was repeated except that di-n-butyl phthalate was used as the solvent of benzotriazole," dimethylformamidewas used as the solvent of silver nitrate and acetone was used as the washing liquid. 17 g. of silver salt of benzotriazole was obtained having an average particle size of 1.0 micron.

j 7 EXAMPLE 4 The same procedure as described in Example 1 was resolvent of benzotriazole, dimethyl sulfoxide was used as the solvent of silver nitrate and methanol was used as the washing liquid. 17 g. of silver salt of benzotriazole was obtained having an average particle size of 0.1 micron.

7 EXAMPLE -The same procedure as described in Example 1 was repeated except that castor oil was used as the solvent of benzotriazole, DMF was used as the solvent of silver nitrate, the temperature during the mixing was 60 C. and ethanol was used as the Washing liquid. 14 g. of silver salt of benzotriazole wasobtained having an average particle size of 0.6 micron.

benzotriazole was 0.1 micron. In this case, the effect of p the surfactant was recognized on the particle size.

EXAMPLE 7 The similar procedure described in Example 1 was repeated except that dibutyl phthalate was used in place of tricresyl phosphate and polyoxyethylene sorbitol'rnonolaurate as a surfactant wasused in place of CmHg and the average particle size of the resulting silver salt of benzotriazole was 1.5; in length.

EXAMPLE 8 The procedure as stated in Example 1 was repeated except that the following surfactant was used in place of C H OSO Na. The average particle size of the thus pre pared silver salt of benzotriazole was 0.05 11. in length.

C zHzs-QSOaNa EXAMPLE 9 The procedure as stated in Example 1 was repeated except that dimethyl. sulfoxide was used in place of water as a solvent for silver nitrate, with the use of 1' g./100 cc. of water of the surfactant C H SO K. The average particle size of the thus prepared silver salt of benzotriazole was 0.08;. in length.

EXAMPLE 10 The procedure as stated in Example 1 was repeated except that dimethyl formamide was used in place of water as a solvent of silver nitrate with the use of 0.5 g./100 cc. of water surfactant The average particle size of the thus prepared silver salt of benzotriazole was 0.5g in length.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A process for producing a silver salt of benzotriazole in the form of finely divided particles, which comprises mixing a solution of silver nitrate dissolved in a solvent A, in which silver nitrate and nitric acid are soluble but in which the silver salt of benzotriazole is insoluble, with a solution benzotriazole dissolved in a solvent 'B, 'in which benzotriazole is soluble but in which the silver salt of benzotriazole and silber nitrate are insoluble, said solvent A being miscible with solvent B in an amount of from 1 to 30% by weight of the totalweight of the solutions A and B, to react said silver nitrate with said benzotriazole to form the silver salt of benzotriazole.

2. The process as claimed in claim 1, wherein the reac- 'vent B is selected from the group consisting of a phosphate, diester of phthalic acid, a diester. of sebacic acid, castor oil, cottonseed oil, linseed oil and tsubaki oil.

' 5. The process as claimed in claim 1,.Wherein said silver nitrate is caused to react with said benzotriazole to form a suspension of said silver salt of benzotriazole and further comprising washing said suspension with a liquid C, said liquid C being miscible with a solvent A and solvent B to thereby forming a three-component homogeneous phase containing solvent A and solvent B in an amount greater than 10%, said silver salt of benzotriazole being substantially insoluble in said liquid C.

6. The process as claimed in claim 1, wherein said silver salt ofbenzotriazole is predominantly present in said solvent B which contains from 1 to 30% by weight, based on the total weight of the solution, of solvent A.

7. The process as claimed in claim 6, wherein said silver salt of benzotriazole is present in said solvent B, which contains from 1 to 30% by weight of the total solution of solvent A, in the form of fine crystals.

' 8. The process as claimed in claim 7 wherein nitric acid is formed as a by-product of said reaction, said nitric acid being present primarily in said solvent A.

9. The process as claimed in claim 1, wherein said solvent B is present in a solvent B rich phase which contains from 1 to 30% by weight of said solvent A, and said solvent B rich phase is separated from said solvent A rich phase subsequent to reaction.

10. The process as claimed in claim 1, wherein said silver salt of benzotriazole is in the form of fine grains which have a length of from 0.1 micron to 3 microns.

11. The process as claimed in claim 10 wherein said fine grains are spindle-like crystals.

12. Theprocess as claimed in claim 3, wherein said phosphate is a trialkyl phosphate selected from the group consisting of triethyl phosphate, tributyl phosphate and octyl-dibutyl phosphate.

13. The process as claimed in claim 3, wherein said di- "ester of phthalic acid is selected from the group consisting of dimethyl phthalate, diethyl phthalate, dipropyl phthalate, dibutylphthalate, di-iso-butyl phthalate, dioctyl phthalate, di-2-ethyl hexyl phthalate and dimethoxyethyl phthalate.

14. The process as claimed in claim 3, wherein said diesterof sebacic acid is selected from the group consisting of diethyl sebacate, dibutyl se'bacate and dioctyl sebacate.

15. The process as claimed in claim 2 wherein the amount of said surfactant is 0.1 to 10 g. per g. of silver nitrate used. i 16. The process of claim 2 wherein said surfactant is selected from thegroup consisting of those represented by the general formula A-SO B, where A is from the group consisting of an alkyl substituted aryl group, an alkoxy group, an alkyl amide-substituted alkylene group and an alkyl substituted carboxyalkyl group, and B is selected from the group consisting of Na or K.

References Cited UNITED STATES PATENTS ,3,252,987 5/1966 Hart j 260-299 NICHOLAS S- RIZZO, Primary Examiner R. I. GALLAGHER, Assistant Examiner US. Cl. X.R. 96-94 

